Nanocomposite materials are comprised of two or more materials, with at least one of the materials including particles having no dimension greater than about several hundred nanometers (nm). Polymer-based nanocomposite materials include a filler material of nanoparticles dispersed in the matrix of the polymer material.
Nanocomposite materials have garnered interest in many technical fields requiring materials for optical, electronic, structural, and barrier applications. One reason for this interest is the potential to combine certain material characteristics of the polymer with those of the filler material. For example, many polymers are transparent and can be used in optical applications. However, in many optical applications it is often useful for the materials to have indices of refraction, or other optical properties, that are different than those of many polymers suitable for optical applications. For example, it would be beneficial to be able to provide a nanocomposite material having a filler material that increases the index of refraction of the material to a desired level, but does not have a significant adverse impact on the transparent nature of the polymer.
Nanocomposites can also be used to provide other material properties. For example, the mechanical properties of tensile strength and compressibility can be altered over those of the unfilled polymer. Beneficially, stronger and more durable materials can be made for structural applications. Moreover, the barrier and thermal properties of the polymer may be altered to a desired end via the incorporation of nanoparticles. Such nanocomposite materials may be used in disparate applications such as construction and packaging.
While polymer-based nanocomposite materials are promising, there are shortcomings in known nanocomposite materials and their methods of manufacture. For example, in order to achieve certain material characteristics and uniformity of these characteristics, it is beneficial for the nanoparticles to be homogeneously dispersed within the other polymer material of the nanocomposite. Often, this requires mixing at the nanometer scale level. However, the nanoparticles and the polymer material often have little or no affinity for one another. Thus, the mixing at the nanometer scale level is difficult to achieve.
Known attempts to address the difficulties presented by the lack of compatibility of the nanoparticles and the polymer material include surface treatment of the nanoparticles prior to mixing with the polymer. For example, U.S. Pat. No. 6,599,631 and U.S. Pat. No. 6,656,990 describe blending a polymer and inorganic particles to form hybrid materials. Specially prepared particles with well-controlled particle size and surface treatment of the particles are required.
Additionally, methods involving mechanical stressing have been used in an attempt to achieve a desired particle size or a more homogeneous dispersion of the particles within the polymer. Known methods include forced mixing using an extruder and injection molding. Unfortunately, such methods may result in the agglomeration of the nanoparticles. Thus, by many known methods of fabricating nanocomposite materials, the domain size of the nanoparticles is on the order of micrometers, which is too great for certain applications.
US Patent No. 2004/004127 to Okubo et al. discloses a polymeric nanocomposite film comprising a cellulose derivative and a polycondensation product of a condensation polymerizable reactive metal compound (but also including compounds containing silicon instead of a “metal” in the conventional sense used herein). The nanocomposite film further contains a plasticizer in an amount of 1 to 20 weight percent by weight. Example 1 of Okuba et al. involves casting a dope comprising titanium tetramethoxide in admixture with methylene chloride, ethanol, silica, and cellulose triacetate polymer. The materials comprise a relatively small amount of the metal alkoxide (less than 4 weight percent) compared to the amount of polymer. Furthermore, as shown in Table 1-1 of the patent, the amount of the polycondensation product in the nanocomposite was not more than 1.5% for metal-oxide nanoparticles, although as high as 20 percent for silicon. Also, as shown in Table 1-2, the average particle size (diameter) of the polycondensation product in the nanocomposite was not less that 90 nm (as measured by small-angle X-ray diffraction) for metal-oxide particles, although nanoparticles as small as 8 nm were obtained when the nanocomposite comprised mixtures of a metallic oxides and silicon oxides, the metallic oxides present in relatively small amounts.
It is believed that the larger size of the metallic oxide nanoparticles in the nanocomposites of Okubo et al., when the polycondensation product consisted only of the metallic oxides, was at least in part due to the relatively higher reactivity of the metallic-oxide precursors compared to the silicon-oxide precursors. Similarly, the lower concentration of the metallic oxides used in the nanocomposites may have been due to agglomeration of the more reactive metal-oxide precursors to the extent that the transparency of the nanocomposite was adversely effected. The concentration of the polycondensation product that could be used by Okubo et al. in the nanocomposites based on metallic oxides was, therefore, severely limited as compared to nanocomposites based on non-metallic oxides such as silicon dioxide.
Significantly, Okubo et al. did not find it necessary to prevent the metal-oxide precursors from reacting in the dope used to make the films. Instead, Okubo et al. state that the condensation polymerization of the reactive metal compound can be carried out in the solution (dope) containing the reactive metal compound or in a web formed on the support, but is preferably carried out in the solution. Evidence supports the conclusion that the reaction of the relatively reactive metal compound in Okubo et al. occurred to a significant extent prior to coating. This is believed to account for the fact that relatively higher concentrations of the metal-oxide nanoparticles (substantially greater than 1.5 weight percent) and/or relatively smaller sizes of the metal-oxide nanoparticles particles (with at least one dimension significantly less than 90 nm) could not be obtained by Okubo et al.
In view of the above, a problem in prior art techniques for fabricating polymer-based nanocomposites is that, with respect to the nanoparticles of a desired material, they may fail to achieve the level of homogeneity desired, the desired loading levels, the desired size of the nanoparticles, or a combination of these deficiencies. Likewise, obtaining polymer-based nanocomposites having desired optical or other properties have been a problem.
It would be desirable to obtain a nanocomposite comprising smaller metallic oxide nanoparticles and/or higher concentrations of the nanoparticulate material in a transparent nanocomposite, particularly a nanocomposite used to make an optical film. What is also needed is a method of fabricating nanocomposite materials that overcomes the shortcomings of the techniques previously discussed.